This invention relates to oscillator arrangements, and is particularly concerned with providing an oscillator output signal with improved frequency stability.
It is well known to provide an oscillator with a resonator in order to determine an oscillation frequency. The resonator may take any of various forms such as crystal, ceramic, dielectric, cavity, and coaxial cable resonators; for brevity herein only crystal oscillators are discussed below but the term xe2x80x9cresonatorxe2x80x9d is used herein to include any of such devices or structures. A crystal resonator may use a rubidium or caesium crystal, or may use any of a variety of piezoelectric materials, such as quartz, employing any of a variety of types of acoustic waves, such as bulk, shallow bulk, and surface acoustic waves, in determining its resonant frequency. The term xe2x80x9ccrystal resonatorxe2x80x9d is used herein to include any such resonators, and in particular to include both bulk wave oscillators and surface acoustic wave (SAW) oscillators. The term xe2x80x9ccrystal oscillatorxe2x80x9d is used herein to mean an oscillator including a crystal resonator.
Crystal oscillators are frequently used to provide an oscillator output signal with a stable frequency. However, it is known that the output signal of a crystal oscillator has a frequency which is dependent upon various parameters such as temperature, acceleration, microphonics, and ageing. It is also known to compensate to some extent for changes of temperature by using a temperature-compensated crystal oscillator, or to reduce temperature changes by placing the crystal resonator and associated circuitry in a temperature-controlled oven.
In some applications a crystal oscillator is required to provide an extremely stable timing or frequency reference. For example, base stations of cellular wireless communications systems need a very accurate timing reference which they derive from GPS (global positioning system) satellites in normal operation, but in the absence of a GPS reference signal they are required to maintain the timing reference to an accuracy of about 7 xcexcs in a 24-hour period. The frequency stability of the oscillator providing this reference must therefore be about 0.08 ppb (parts per billion).
For such frequency stability, sensitivity to acceleration is not significant because the crystal oscillator is stationary, microphonics can be avoided with good mechanical design, and ageing over the 24-hour period is very small, for example less than 0.02 ppb, so that temperature sensitivity is a dominant factor. A temperature compensated crystal oscillator may provide a frequency stability of only about 1 ppm (part per million), an ovenized crystal oscillator may provide a greater but still inadequate frequency stability, and a doubly ovenized crystal oscillator may still only provide a frequency stability of about 0.4 ppb; the steps of temperature compensation, ovenizing, and double ovenizing add considerable and successively increasing costs.
It is known from Onoe U.S. Pat. No. 3,978,432 issued Aug. 31, 1976 and entitled xe2x80x9cOscillator Having Plural Piezoelectric Vibrators Parallel Connected For Temperature Compensationxe2x80x9d to provide an oscillator with two or more resonators connected in parallel, at least one of the resonators having a second-order frequency-temperature characteristic and the others each having a third-order frequency-temperature characteristic with resonant frequencies selected to provide a wider compensated temperature range for the oscillator. However, such an arrangement would be very difficult to provide in practice and does not provide frequency stability over an extended temperature range.
It is also known from Hartemann U.S. Pat. No. 4,338,575 issued Jul. 6, 1982 and entitled xe2x80x9cProcess For Compensating Temperature Variations In Surface Wave Devices And Pressure Transducer Utilizing This Processxe2x80x9d to provide two SAW delay line oscillators with closely similar frequencies which are mixed to produce an output frequency that is a function of a parameter, such as force, pressure, or acceleration, to be measured, and to include in one of the oscillator loops an extra temperature-dependent delay for temperature compensation. In this arrangement the delay-temperature characteristic of the extra delay must precisely compensate for drift between the SAW delay lines with changing temperature; the reference assumes this to be linear, an assumption that is not necessarily valid. Further, the reference is concerned primarily with difference measurements for which an increase in output frequency is generally unimportant, and otherwise suggests additional measures. Such additional measures are not practicable to achieve the frequency stability as discussed above.
Accordingly, an improved oscillator arrangement is required to provide the desired frequency stability. Even where such extremely high frequency stability is not required, it is desirable to provide an oscillator arrangement with improved frequency stability, for example to reduce requirements for temperature compensation or ovenizing at any desired level of frequency stability.
According to one aspect, this invention provides an oscillator arrangement comprising: a first oscillator, comprising a resonator for determining a frequency of the oscillator, for producing a first signal at a frequency f1 having a dependence upon a predetermined parameter P which includes a term c1Pn where c1 is a coefficient and n is a non-zero integer; a second oscillator, comprising a resonator for determining a frequency of the oscillator, for producing a second signal at a frequency f2 having a dependence upon the predetermined parameter P which includes a term c2Pn where c2 is a coefficient not equal to c1; and a mixer for combining the first and second signals to produce an output signal of the oscillator arrangement at a frequency f1xe2x88x92f2; wherein f2=(c1/c2)fl whereby the output signal frequency has substantially zero dependence on Pn.
For example, the oscillators can comprise crystal oscillators the resonators of which comprise surface or bulk acoustic wave devices.
The first and/or second oscillator can include a frequency divider for producing the first and/or second signal, respectively, from an oscillation frequency of the oscillator determined by the resonator.
In particular, the predetermined parameter P can comprise temperature. For example, the resonators of the oscillators can be selected to have a predominantly linear dependence of frequency upon temperature, with n=1, or they can be selected to have a substantially zero first-order dependence of frequency upon temperature, with n=2, or they can be selected to have a predominantly third-order dependence of frequency upon temperature, with n=3.
A cascade of such oscillator arrangements can be provided in order to reduce temperature dependence for different powers of temperature, i.e. for a plurality of values of n. Thus the invention also provides an oscillator arrangement comprising a first oscillator arrangement as recited above for which the first and second oscillators comprise second and third, respectively, oscillator arrangements each as recited above, n having a first value for the first oscillator arrangement and a second value, different from the first value, for both of the second and third oscillator arrangements.
Such an oscillator arrangement can include one or more frequency dividers each for producing a signal frequency supplied to a respective mixer from a respective oscillator. The mixers and frequency dividers are conveniently constituted by a programmable digital circuit.
Another aspect of the invention provides an oscillator arrangement comprising two oscillators for producing respective signals at two different frequencies each of which has a respective dependence upon a parameter in accordance with a polynomial with respective coefficients which are different for the two oscillators, a product of one of the frequencies and a selected one of the coefficients of its polynomial being substantially equal to a product of the other frequency and a corresponding coefficient of its polynomial, and a mixer for producing a sum or difference frequency of the two signals for which a corresponding coefficient of a respective polynomial is substantially zero.
A further aspect of the invention provides a method of producing a signal at a desired frequency, comprising the steps of: producing two signals at respective frequencies using two oscillators, said respective frequencies being dependent upon temperature of the oscillators in accordance with respective polynomials having coefficients that are different for the respective frequencies, the respective frequencies having a frequency sum or difference equal to the desired frequency and a ratio which is inverse to a ratio of a selected one of the coefficients of the respective polynomials; and mixing said signals at the respective frequencies to produce the signal at the desired frequency with a dependence upon temperature in accordance with a polynomial for which the selected one of the coefficients is substantially zero.